1. Field of the Invention
This invention relates to a device and method for tapping optical energy from an optical fiber.
2. Description of the Prior Art
Optical fibers are used increasingly for transmitting data and other communications in a wide variety of situations. Both single mode and multimode fibers are used for relatively long-distance communications between telephone central offices or other installations at distances of several kilometers. In addition, optical fibers are being used for relatively shorter distance communications, such as connecting campus-type office complexes, connecting computers and computer terminals, or connecting components in an electronic switching system for telephone and other telecommunication purposes. It is expected that in automated office systems, various terminals, including printers, display screens, etc., will be connected by means of optical fibers. Some of the envisioned applications--for example, tapping television or other video signals or tapping high-speed data transmission from an optical fiber--require that a tap be able to handle wide bandwidth signals or high data rates. In other cases, the bandwidth is not so significant as the ability to conveniently insert or remove a tap. It is especially desirable to be able to tap an optical fiber without breaking the fiber and without removing the coating. Furthermore, it is desirable to be able to change the amount of signal tapped from the fiber, for use with devices having different sensitivities, or to compensate for the fiber attenuation of optical energy as a function of distance from the source. Finally, it is desirable that a tap introduce minimal loss to the optical energy propagating through the fiber.
Some previously reported optical fiber taps require one or more connectors in an optical fiber system. That technique typically necessitates breaking the fiber to make a tap. In addition, the system is disrupted in some cases when a tap is connected or removed. Connector losses of 0.2 to 2.0 dB are typically inherent in these designs, limiting the number of taps that can be connected in a fiber distribution system. In one case, optical energy scattered from the end of a fiber was collected; see "Optical-Fibre Tap With Low Insertion Loss," F. R. Gfeller et al, Electronics Letters, Vol. 15, pages 448-450 (1979).
In other optical fiber taps, breaking a fiber or attaching a connector is not necessary, with the optical energy being removed by coupling power to modes that radiate out of the fiber to a detector; see, for example, U.S. Pat. Nos. 3,931,518, 3,936,631, and 3,982,123, coassigned with the present invention. The generic technique disclosed and claimed therein is to radiate light out of the core of the fiber into the cladding, as by bending the fiber. The light is then coupled out of the fiber cladding by a dielectric body, and detected by means of a detector. While the embodiments shown in the abovenoted patents are useful in many situations, there is a need for further improvement for many applications.